The present invention relates to a dual hard disk drive system for dropped write detection and recovery.
Computer systems often require a considerable amount of nonvolatile disk storage to preserve software, programs and other data that cannot fit in the smaller more costly RAM memory and that otherwise would be lost when the systems are turned off. At present, it is common for these storage systems to be built using a large number of Hard Disk Drives (HDDs). HDDs are constructed using one or more disk shaped platters coated with a magnetic material. The disk platters spin at fixed speeds and a movable arm with a read/write head is directed to specific locations on the disk to write and/or read data. The head assembly glides just above the surface of the platter. During data write operations, the head assembly applies an electric field to a specific location on the disk creating a substantially permanent magnetic field in a specific direction. If the field points in one direction it represents a binary “1” and if it points in the other direction is represents a binary “0”. The head assembly is designed to read stored data by sensing the small current induced in the head assembly by the magnetic field when in passes over the magnetized location on the platter. When the HDD is powered off, the data is preserved by the magnetic signature.
HDD platters are partitioned into concentric circles, called tracks, which are coincident with areas over which the head glides when the arm assembly remains motionless. Each track is further partitioned into sectors. Each sector contains a larger fixed length area for data as well as header and trailer information used by the HDD electronics during the data storing and retrieval process. Data read and write times, called latency, are not fixed and predictable as they are in RAM. The latency, to a large extent, is a function of the seek time, the time it takes the arm to reposition the head over the track where the data is to be stored or retrieved. That time is variable and a function of the last position of the arm.
HDDs are typically designed as self contained assemblies that can be plugged into standard slots in computer chassis or in a separate storage chassis. Separate storage drawers typically hold anywhere from a half dozen to as may as 50 or more individual HDDs. A storage chassis can be either a stand-alone assembly or a rack mountable unit to allow multiple drawers to be placed into a single rack creating a relatively large array of HDDs in a small physical foot print. Drive density per unit area floor space is a competitive metric used in the industry to help potential customers compare offerings from different vendors.